RFID-systems are employed in various fields of automatic identification. Especially in the field of industrial automation the use of RFID-systems proves to be of advantage. The growing requirements of quality management and quality assurance in automatic production can easily be realized with RFID-systems. In the production process production and measurement data can easily be stored on the object itself and taken alongside. In this way the data are available and can be documented on each object at any time. This also allows for a greater flexibility in production. Production parameters can be stored on the object and read out directly in the respective production line. Objects can be removed from the series at any chosen time and added again at a later point in time without any errors being caused. Due to the decentralized data storage on RFID-tags even a breakdown of the central computer does not pose any problem with regard to the clear identification and localization of the objects.
Basically, RFID-systems are distinguished between coupled systems and microwave systems. In the coupled systems data exchange between the reader unit and the tag takes place through capacitive or inductive coupling. As a rule, these systems operate at the frequencies of 135 kHz or 13.56 MHz. In microwave systems a non-reacting, electromagnetic coupling takes place via the far field of an antenna. In this case use is made of the frequency ranges 860-930 MHz, 2.45 GHz as well as 5.8 GHz.
An essential part of the available RFID-systems functions according to the so-called backscatter method. In these systems the reader unit transmits a constant, sinusoidal carrier signal (continuous wave, CW) that supplies energy to the tag. In addition, a part of the carrier signal is reflected on the tag. Through a change of the antenna backscattering characteristics the tag modulates the data to be transmitted onto the carrier signal, in which case the data can then be recognized in the frequency spectrum as sidebands around the signal.
Based on the known equations for field propagation the essential receiving and transmission power of a transmission link can be described as follows:    1. Transmission loss ardB in decibel as a function of the distance r and the wavelength λ (λ=c/f):
      a    r    dB    =            -      10        ·                            log          ⁡                      (                          λ                              4                ⁢                π                ⁢                                                                  ⁢                r                                      )                          2            .          2. Receiving power according to the tag antenna PtedBm in dBm as a function of the transmission power of the reader unit PrsdBm in dBm, the gain of the transmission antenna GsdB in decibel, the gain of the tag antenna GtdB in decibel and the transmission loss ardB in decibel:PtedBm=PrsdBm+GsdB+GtdB−ardB.    3. Transmission power of the tag PtsdBm in dBm as a function of the receiving power of the tag and the modulation losses on the tag PmoddB in decibel:PtsdBm=PtedBm−PmoddB.    4. Receiving power on the reader unit PredBm in dBm as a function of the described parameters and:
                              P          re          dBm                =                ⁢                              P            ts            dBm                    +                      G            s            dB                    +                      G            t            dB                    -                      a            r            dB                                                  =                ⁢                              P            rs            dBm                    +                      2            ⁢                          G              s              dB                                +                      2            ⁢                          G              t              dB                                -                      2            ⁢                          a              r              dB                                -                                    P              mod              dB                        .                              
In addition, a system dynamic D required for the distance can be indicated in decibel by means of the difference between the transmission power PrsdBm and the receiving power PredBm of the reader unit:DdB=PrsdBm−PredBm.
In the industry standards ISO-IEC 18000-6C and ETSI EN 302 208-1 the stipulated ruling for the UHF RFID-systems is in the frequency range of 860 MHz-930 MHz. In this, a maximum transmission power of 2 W ERP (equivalent radiated power) and the modulation methods to be used are stipulated for example.
Hence, for a link of one meter a transmission loss of approximately 31.2 dB ensues, which increases by a factor of 6 dB with each doubling of the transmission link. Under realistic conditions:                Transmission power 2 W ERP,        Operating frequency 866.5 MHz,        Antenna gain of the reader antenna and the tag antenna of 6 dBi and 1 dBi, respectively,        Modulation losses of 6 dB        and reflection losses of 0.5 dBan input power of approximately −19.7 dBm with a corresponding necessary system dynamic of 55.9 dB as a difference between the transmission power and the receiving power is thereby resultant for this distance. Each doubling of the distance leads to a reduction of the input power by 12 dB with a corresponding requirement of a 12 dB increased system dynamic.        
In the backscatter method transmission of the carrier signal and reception of the tag response take place simultaneously (duplex operation). This can be realized by making use of a circulator, as described for example in FIG. 1. In the case of such a circulator crosstalk can occur between the transmission connection and the reception path, which increases upon a decreasing isolation between transmission path and reception path. The amount of crosstalk or of the isolation constitutes a significant system characteristic that contributes considerably to the system dynamic. In commercial circulators the crosstalk lies in the range of 20 to 30 dB.
Crosstalk can also ensue from reflections of the transmission signal on the common transmission/reception antenna, which are passed on by the circulator in accordance with its function to the reception path. Such reflections can be based for example on minor mismatches of the antenna.
As a result of the crosstalk tag responses may be superimposed so that they can no longer be demodulated without error. For this reason measures for reducing the crosstalk have been taken into consideration.
Known approaches for the reduction of the carrier signal level in the reception path are primarily restricted to the suppression of crosstalk between the transmission and the reception path. Such approaches firstly include the use of two separate antennas for transmitting and receiving instead of a circulator and a combined transmitting/receiving antenna. Here, by an optimal arrangement of the individual antennas as well as by application of damping elements at appropriate places of the respective antenna housings, crosstalk between the transmission and the reception path can be further reduced. In practice, values hereby amount to approximately −40 dB.
The disadvantage of this procedure resides in the sensitivity of the crosstalk to environmental influences in the vicinity of the system or the tag. Such environmental influences are e.g. larger reflecting areas which lead to a high transmission factor between the transmission and the reception antenna, which may interfere with the weak tag response as a disturbing signal.
Furthermore the circuit may be expanded by a fixed component connected in parallel, which together with the equivalent circuit parameter of the crosstalk constitutes a resonance circuit with a resonance frequency at the operating frequency. Thus, the crosstalk for the operating frequency may be compensated by some decades, depending on the performance of the component used. This approach may be realized with application of a few SMD components and is inexpensive.
A disadvantage of this procedure initially lies in the invariant frequency range of the compensation whereby the application is restricted to a single frequency. Furthermore, compensation may only be achieved to a small extent on account of the discrete values of the available components. With respect to modifications of the transmitting and/or receiving antenna, the compensation remains comparatively inflexible. The compensation after all is narrow-band for which reason a strong dependency of the absolute frequency position of compensation on the manufacturing tolerances arises. This leads to the fact that the improvement of the isolation actually achieved at the operating frequency can vary considerably.
By using components with variable values such as varactor diodes, a certain flexibility regarding the frequency position and the compensation may be introduced. In practice, values herein are approximately −35 dB.
There are furthermore only a few approaches to carry out a suppression of the carrier signal in the receiving path. By using a digital signal processing, e.g. by DSPs (Digital Signal Processors), a narrow-band suppression of up to −80 dB is possible. Furthermore, when taking this approach frequency selective filters with differing band widths may be realized.
The disadvantage of a digital signal processing on the one hand is the higher requirement for capital expenditure both for the acquisition of hardware and for the development time required for software development as well as the respective costs incurred herein. On the other hand the hardware requires quite some space and is, depending on the respective requirements to be fulfilled, considerably slower than an analogue circuit.
A further possibility to achieve an improved suppression of the carrier signal in the reception path is described in “System combining radio frequency transmitter and receiver using circulator and method for cancelling transmission signal thereof” (U.S. Pat. No. 6,567,648 B1). Herein, a suppression of crosstalk is achieved by decoupling a part of the receiving signal (“Rx+Tx” in FIGS. 3 and 4) and modifying it in phase and amplitude in such a way that the fractions of the carrier signal in the original receiving signal and in this decoupled signal path, the latter having the same amplitude but being shifted in phase by 180°, can be brought together. Herein, a filtering of the Rx-Signals becomes necessary (components 24 in FIGS. 3 and 4), which—due to the small frequency offset between the carrier signal and the tag response in the backscatter procedure in RFID systems—could only be achieved by means of a digital signal processing.
In accordance with the state of the art, merely by digital signal processing the carrier signal can be effectively suppressed for individual channels. The requirements for filtering an individual operating channel are so high (the relative bandwidth in UHF RFID for instance is 0.23%) that in turn only a digital signal processing allows a reliable filtering of an individual operating channel.
Circulators, directional couplers and variable phase shifters are described in U.S. Pat. No. 6,603,391 B1, WO 2006/088583 A2, US 2006/0098765 A1, and US 2008/0081551 A1, respectively.